Solar cells provide a renewable energy source that can be implemented in a wide variety of geographic regions. However, solar cells account for only a small percentage of energy production (for example, in the United States, solar energy accounted for 0.065% of energy production in 2005, a figure which includes both photovoltaic and solar thermal energy). The most common type of photovoltaic cell is made from silicon; however, preparing the high-purity silicon required for their manufacture is costly both in economic terms and the energy input required to purify the silicon. Thus silicon-based solar cells are used primarily in remote locations or in markets where the importance of ecological sustainability outweighs the cost of inorganic solar cells.
Organic thin-film-based solar cells, such as polymer-based solar cells, have been the subject of much research as alternatives to the high-cost inorganic solar cells. These solar cells are typically fabricated with an electron donor material and an electron acceptor material, which allows an electron-hole pair (an exciton) generated by a photon to separate and generate current. The junction between the donor and acceptor can be created by forming a layer of one material (e.g., the donor) on top of the other material (e.g., the acceptor), which forms a planar heterojunction between the bilayers. Since the planar bilayer heterojunction affords a relatively small area for charge separation to occur, different morphologies have been explored. Interpenetrating networks of donor and acceptor materials can be used; these range from diffuse interfaces at a bilayer heterojunction to bulk heterojunctions, where the donor and acceptor materials are mixed together and form a multi-component active layer.
Bulk heterojunction (BHJ) solar cells can be fabricated from blends containing a conjugated polymer and a fullerene derivative and have the potential to generate inexpensive, flexible, photoconductive devices such as photodetectors and solar cells, avoiding the cost constraints of silicon-based devices. A major advantage of such plastic solar cells rests in their ability to be processed from solution, a feature which may make polymer-based devices more economically viable than small-molecule based organic photovoltaic cells.